Method of selecting refrigerant-lubricant combinations

ABSTRACT

The present invention provides methods for selecting refrigerant and lubricant combinations for use in heat transfer cycle systems and provides methods for operating said heat transfer systems. More particularly, the invention provides methods to select lubricant and refrigerant combinations for a heat transfer cycle system wherein at the lower temperatures of the heat transfer cycle the refrigerant and lubricant are miscible and at the upper temperatures of the heat transfer cycle the refrigerant and lubricant are phase separated and such that the density phase inversion temperature of the combination is below the upper operating temperature of the heat transfer cycle.

This application is continuation of U.S. patent application Ser. No.13/517,082, filed Jun. 19, 2012, which is the United States nationalphase of and claims priority to International Application serial numberPCT/US2010/061258 filed Dec. 20, 2010, which designated the UnitedStates, which claims priority to U.S. provisional application Ser. No.61/290,690 filed Dec. 29, 2009, all of which are incorporated herein byreferences.

FIELD OF THE INVENTION

The present invention relates to methods for selecting refrigerant andlubricant combinations for use in heat transfer cycle systems andprovides methods for operating said heat transfer systems. Moreparticularly, the invention relates to methods used to select lubricantsand refrigerants combinations for a heat transfer cycle system whereinat the lower temperatures of the heat transfer cycle the refrigerant andlubricant are miscible and at the upper temperatures of the heattransfer cycle the refrigerant and lubricant are phase separated andsuch that the density phase inversion temperature of the combination isbelow the upper operating temperature of the heat transfer cycle.

BRIEF SUMMARY OF THE INVENTION

The miscibility between refrigerants and lubricants in heat transfersystems, such as vapor-compression refrigeration systems, is importantin determining the performance of such systems. In many cases,miscibility between the lubricant and refrigerant at all conditions ofoperation is preferred in order to ensure adequate lubrication,sufficient circulation of lubricant in the system, and maintainefficient heat transfer in critical system components. However, manycombinations of refrigerants and lubricants exhibit phase separation atelevated temperatures. A method of selecting refrigerant and lubricantcombinations has been discovered that uses such phase separatingcombinations of refrigerants and lubricants that also exhibit theproperty of density phase inversion. At temperatures below the “phaseinversion temperature” the lubricant-rich phase is less. dense than therefrigerant-rich phase while at temperatures above the “phase inversiontemperature” the refrigerant-rich phase is less dense than thelubricant-rich phase. The temperature at which a particular combinationflips from one phase denser to the other, and thereby one phase on topto the other phase, is the phase inversion temperature.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention is a method of selectingrefrigerant and lubricant combinations in a heat transfer system,including but not limited to refrigeration vapor-compression-typesystems, that incorporates at least one refrigerant and at least onelubricant. In a further embodiment of the present invention, the methodof selecting refrigerant and lubricant combinations is such that a loweroperating temperature range is selected where the refrigerant andlubricant form a single phase and an upper operating temperature rangeis selected where the refrigerant and lubricant phase separate intorefrigerant-rich and lubricant-rich phases. In a further embodiment, therefrigerant-lubricant phase inversion temperature is between the loweroperating temperature and upper operating temperature ranges. In atraditional refrigeration vapor-compression system, such as inrefrigeration or air conditioning systems, such lower and upperoperating temperatures may be the evaporator discharge temperature andcompressor discharge temperature respectively.

The present method of selecting refrigerant and lubricant combinationswill promote efficient oil return in the heat transfer system and helpmaintain efficient performance. For example, in the evaporator of arefrigeration vapor-compression cycle, immiscibility between therefrigerant and lubricant can lead to oil logging and excess oil holdup, which can insulate the evaporator to heat transfer, reducing thesystem efficiency. If the lubricant and refrigerant are miscible, thelubricant tends to be passed from the evaporator and returned back tothe compressor. Therefore, a preferred embodiment of the presentinvention is where the refrigerant and lubricant combination is miscibleat the lower temperatures of the heat transfer system to minimize oillogging.

Another issue is the potential for oil collecting in the receivers orreservoirs of heat transfer systems. Typically, if the lubricant andrefrigerant are immiscible, a lubricant-rich phase may float on thesurface of the denser refrigerant-rich phase in receivers or reservoirs,this places the lubricant-rich phase above the suction line for returnto the compressor. This can lead to collection of the lubricant in thesereservoirs and subsequent drainage of lubricants from the compressor,which can result in insufficient lubrication of the compressor partsleading to excessive wear and premature failure. If the temperature ofsuch reservoirs or receivers is sufficiently above the phase inversiontemperature of the refrigerant-lubricant combination, the lubricant-richphase will sink to the bottom and be collected back to the compressor,thereby ensuring efficient oil return. If the temperature in suchreservoirs or receivers is at or near the phase inversion temperature ofthe refrigerant-lubricant combination, then the lubricant-rich andrefrigerant-rich phases can form stable emulsions or mixtures with longsettling times, again permitting efficient oil return. An embodiment ofthe present invention is that where the phase inversion point is betweenthe upper and lower operating temperatures of the system and preferablynear or below the temperature of the receiver or reservoirs such that alubricant-rich upper layer phase does not form in the receivers orreservoirs.

The miscibility behavior of the refrigerant and lubricant is veryimportant in the compressor of the heat transfer system, where thelubrication is essential for proper operation and maintaining equipmentlife. If dissolved refrigerant decreases the viscosity of the lubricanttoo much, there may be insufficient lubrication in the compressorleading to excessive wear and premature failure. Another concern is thatof flooded starts, where the compressor sump is flooded with refrigerantafter shutdown. During startup, the presence of refrigerant can reducelubricant viscosity resulting in inadequate compressor lubrication. Thisis particularly a concern with immiscible refrigerant/lubricant systemswhere two layers can form in the compressor sump with the refrigerantlayer on the bottom. The withdrawal point where lubricant is normallydrawn into the compressor bearings is at the bottom. In a preferredembodiment of the present invention, the temperature in the compressoris above the phase inversion temperature, thereby limiting the chancethat refrigerant will displace lubricant in the compressor sump whichwill aid lubrication and avoid many problems of flooded starts. In aseparate embodiment, the temperature in the compressor during idleperiods is such that the refrigerant and lubricant are miscible.

Though not meant to limit the scope of the present invention,refrigerants of the present invention include compositions comprisinghydrofluorocarbons (HFCs), hydrofluoroolefins (HFOs),hydrochlorofluorocarbons (HCFOs), hydrochlorofluorocarbons (HCFCs),hydrocarbons (HCs), carbon dioxide, ammonia, dimethyl ether, and/ormixtures thereof. Preferably the refrigerant comprises ahydrofluoroolefin. More preferably, the a fluorinated C3 to C6 alkene,more preferably a fluorinated C3 to C4 alkene, even more preferably atri-, tetra-, or pentafluoro-propene, and even more preferably atrifluoropropene, tetrafluoropropene, and/or mixtures thereof. Exemplarytrifluoropropenes include 3,3,3-trifluoropropene (HFO-1243zf). Exemplarytetrafluoropropenes include 2,3,3,3-tetrafluoropropene (HFO-1234yf) and1,3,3,3-tetrafluoropropene (HFO-1234ze). 1,3,3,3-tetrafluoropropene(HFO-1234ze) can include the cis-isomer, trans-isomer, and mixturesthereof; preferably 1,3,3,3-tetrafluoropropene is predominantly thetrans-isomer.

Exemplary HFCs include, hut are not limited to, difluoromethane(HFC-32); 1-fluoroethane (HFC-161); 1,1-difluoroethane (HFC-152a);1,2-difluoroethane (HFC-152); 1,1,1-trifluoroethane (HFC-143a);1,1,2-trifluoroethane (HFC-143); 1,1,1,2-tetrafluoroethane (HFC-134a);1,1,2,2-tetrafluoroethane (HFC-134); 1,1,1,2,2-pentafluoroethane(HFC-125); 1,1,1,3,3-pentafluoropropane (HFC-245fa);1,1,2,2,3-pentafluoropropane (HFC-245ca); 1,1,1,2,3-pentafluoropropane(HFC-245eb); 1,1,1,3,3-hexafluoropropane (HFC-236fa);1,1,1,2,3,3,3-heptafluoropropane (HFC-227ea);1,1,1,3,3-pentafluorobutane (HFC-365mfc),1,1,1,2,3,4,4,5,5,5-decafluoropropane (HFC-4310), and mixtures thereof.Exemplary hydrocarbons include, but are not limited to, propane, butane,isobutane, propylene, and mixtures thereof. Exemplaryhydrochlorofluorocarbons include, but are not limited to, HCFC-22,HCFC-123, and mixtures thereof. Exemplary HCFOs include, but are notlimited to, 1-chloro-3,3,3-trifluoropropene (HCFO-1233zd), preferablythe trans-isomer, 2-chloro-3,3,3-trifluoropropene (HCFO-1233xf), and/ormixtures thereof.

Though not meant to limit the scope of the invention in any way,lubricants of the present invention, include but are not limited to:polyalkylene glycols (PAGs), polyol esters (POEs), polyvinyl ethers(PVEs), polyglycols, polyalkylene glycol esters, alkyl benzenes, mineraloils, polyalphaolefins, and/or mixtures thereof. PAG oils are preferablyhomopolymers or copolymers consisting of two or more oxypropylenegroups. PAG oils can be ‘un-capped’, ‘single-end capped’, or ‘double-endcapped’. Examples of commercial PAG oils include, but are not limitedto, ND-8, Castrol PAG 46, Castrol PAG 100, Castrol PAG 150, DaphneHermetic PAG PL, Daphne Hermetic PAG PR. Example commercial POE oilsinclude, but are not limited to, Emkarate POE RL 32H, Emkarate POE RL68H, Copeland Ultra 22CC, Copeland Ultra 32CC. The preferred viscosityof the lubricating oil is from about 10 to about 200 centistokes.

Example of refrigerant/lubricant combinations within the scope of thepresent invention include:

HFO-1234yf/PAG

HFO-1234yf/POE

HFO-1234yf/PVE

HFO-1234ze/PAG

HFO-1234ze/POE

HFO-1234ze/PVE

HFO-1243zf/PAG

HFO-1243zf/POE

HFO-1243zf/PVE

It is recognized the chemical composition of lubricants within the samefamily (eg. PAGs) can be different. Therefore, the miscibility behaviorand inversion temperature can be different from one lubricant to thenext even when used in combination with the same refrigerant. Therefore,the operation temperatures of the method of cooling of the presentinvention may change depending on the specific combinations ofrefrigerant and lubricant. A person of ordinary skill in the art wouldbe able to determine the phase inversion temperature of a specificcombination.

Heat transfer systems, including for refrigeration, air conditioning,and Liquid chilling, are operated with one portion of the cycle at a thelower operating temperature range and another part of the cycle at theupper operating temperature range. These upper and lower temperatureranges will depend on the specific application. For example, theoperating temperatures for low temperature refrigeration may bedifferent than for automotive air conditioning or for water chillers.Preferably, the upper operating temperature range is from about +15° C.to about +90° C., more preferably from about +30° C. to about +70° C.Preferably, the lower operating temperature range is from about +25° C.to about −60° C., more preferably from about +15° C. to about −30° C.For example, a low pressure liquid chiller may be operated at anevaporator temperature from about −10° C. to +10° C. and a condensertemperature from about +30° C. to +55° C. For example, an airconditioner, such as for automotive AC, may operate with an evaporatingtemperature at 4° C. and a condensing temperature of 40° C. Forrefrigeration, the lower operating temperature range may be depend uponthe specific application. For instance, some typical applicationtemperatures for refrigeration include: freezer (eg. ice cream): −15°F.+/−2° F. (−26° C.+/−1.1° C.); low temperature: 0° F.+/−2° F. (−18°C.+/−1.1° C.); medium temperature: 38° F.+/−2° F. (3.3° C.+/−1.1° C.).These examples are only informative and not meant to limit the scope ofthe present invention in any way. Other operating temperatures andoperating temperature ranges may be employed within the scope of thepresent invention.

The compositions may also comprise additives, such as dyes, viscositymodifiers, anti-foaming agents, corrosion inhibitors, stabilizers,compatibilizers, anti-oxidants, pour point depressants, nanoparticles,flame suppressants and mixtures thereof.

Another embodiment of the present invention is a method of operating aheat transfer system with a refrigerant-lubricant combination. In oneembodiment, one portion of the heat transfer system is operated at alower operating temperature range where the refrigerant and lubricantform a single phase. Another portion of the heat transfer system isoperated at an upper operating temperature range where the refrigerantand lubricant separate into a refrigerant-rich phase and alubricant-rich phase. In this embodiment of the present invention, theheat transfer system is operated where the density phase inversiontemperature of the refrigerant/lubricant combination is between theupper and lower operating temperatures.

Example 1

To a graduated pressure vessel was added approximately equal parts ofHFO-1234yf and a commercial PAG lubricant (Castrol PAG 46). Thecomponents were mixed and allowed to sit at ambient temperature toequilibrate. At ambient temperature (˜20° C.), the mixture was phaseseparated, with a lubricant-rich liquid phase floating on arefrigerant-rich liquid phase. To reduce the temperature of the mixturebelow ambient, the pressure vessel was placed in a contain temperaturerefrigerator or freezer and cooled until equilibrium was reached. At 8°C. the mixture was miscible, showing only a single liquid phase. At −20°C. the mixture was miscible with only a single liquid phase.

To raise the temperature of the mixture above ambient, the graduatedpressure vessel was then placed in a constant temperature bath andheated in stages to from 25° C. to 50° C., allowed to reach equilibriumat each stage, and the vessel contents were periodically observed. Themixture contained two liquid phases. At 30° C. the two phases were verydifficult to distinguish. At 35° C., the two phases were still difficultto distinguish but a liquid, refrigerant-rich phase was floating above amore lubricant-rich phase. At 40° C. and higher, the portion of thesample that was the refrigerant-rich phase had increased and the twophases were more easily distinguished.

The graduated pressure vessel was removed from the constant temperaturebath and allowed to cool. As the vessel cooled, the refrigerant-richphase was observed to sink to the bottom of the vessel.

This example shows that the combination of HFO-1234yf with Castrol PAG46 is miscible from 8° C. to −20° C. while being immiscible at +20° C.and above, with a density phase inversion temperature of about +30° C.

Example 2

A vapor-compression air conditioning system can be operated using therefrigerant/lubricant combination of Example 1, where the refrigerant isHFO-1234yf and the lubricant is Castrol PAG 46. The lower operatingtemperature range could be from about +8° C. to about −20° C., while theupper operating temperature range could be from about 30° C. and above.The air conditioning system would be operated according to the presentinvention, where the evaporator temperature is maintained at 4° C.+/−2°C., and the condensing temperature is maintained at about 40° C. Usingthese operating conditions, the refrigerant and lubricant would bemiscible at the coldest conditions in the air conditioning system whileat the upper operating temperatures, the refrigerant and lubricant wouldbe immiscible but where the refrigerant-rich phase is less dense.

The invention claimed is:
 1. A method of operating a vapor-compressionheat transfer system containing a refrigerant and a lubricantcombination comprising: a. determining a lower, evaporator dischargeoperating temperature range of a vapor-compression heat transfer system;b. determining an upper, compressor discharge operating temperaturerange of the vapor-compression heat transfer system; and c. wherein saidrefrigerant is selected from hydrofluoroolefins (HFOs) selected from thegroup consisting of 3,3,3-trifluoropropene (HFO-1243zf),2,3,3,3-tetrafluoropropene (HFO-1234yf) and 1,3,3,3-tetrafluoropropene(HFO-1234ze), or mixtures thereof at a first concentration and saidlubricant is selected from polyalkylene glycols (PAGs), polyol esters(POEs), polyvinyl ethers (PVEs), polyglycols, polyalkylene glycolesters, alkyl benzenes, mineral oils, polyalphaolefins, or mixturesthereof at a second concentration and wherein said refrigerant and saidlubricant are miscible at a first temperature within said lower,evaporator discharge operating temperature range of about −60° C. toabout +25° C., and produce a fluid system having a refrigerant-richphase and a lubricant-rich phase at a second temperature within saidupper, compressor discharge operating temperature range of about +15° C.to about +90° C., provided that said second temperature is higher thansaid first temperature, wherein the lubricant-rich phase has a higherdensity than the refrigerant-rich phase at said second temperature andwherein a phase inversion temperature is between the lower, evaporatordischarge operating temperature range and the upper, compressordischarge operating temperature range.
 2. The method of claim 1 whereinsaid upper, compressor discharge operating temperature range is about+30° C. to about +70° C. and said lower, evaporator dischargetemperature range is about −30° C. to about +15° C.
 3. The method ofclaim 1 wherein said fluid system further comprises one or more ofadditives selected from the group consisting of dyes, viscositymodifiers, anti-foaming agents, corrosion inhibitors, stabilizers,compatibilizers, anti-oxidants, pour point depressants, nanoparticles,flame suppressants and mixtures thereof.